The detection range of an electronic article surveillance (EAS) tag that is based on resonant resistor-inductor-capacitor (RLC) circuits is limited by the magnetic flux that is available at the coil or inductor of the tag from the reader transceiver. This is particularly problematic for RF tags designed to generate frequency division signals so as to divide a broadcast or carrier frequency. These tags generally require a minimum RF field in order to activate the frequency division signal. Further, Federal Communications Commission (FCC) regulations on the amount of RF field that can be broadcast by a tag reader also limit the read range, a key specification parameter for an RF EAS or RFID system.
One way to generate a frequency division signal in a tag is to have a circuit containing an inductor in parallel with a nonlinear capacitance element, such as a MOS capacitor or diode, which is tuned to a frequency that is a factor of two away from the broadcast or carrier frequency. Since the nonlinear tag resonance or resonant frequency is away from the carrier frequency, the amount of signal coupled into the nonlinear coil may be relatively low.
In other approaches, the local flux and carrier signal coupling into the tag can be greatly enhanced by employing two RLC circuits in the tag, such that: (1) the first resonates at the reader broadcast frequency (F); (2) the second resonates at the reader sensing frequency (F/N), where N is typically 2, but N can be an integer higher than 2; and (3) the coils of the two RLC circuits are in close proximity to each other so that the resonance enhancement of the reader broadcast field by the first RLC circuit couples effectively into the second RLC circuit. However, due to coupling and mutual inductance effects, bringing two inductive coils into close proximity may shift the resonances to new frequencies as compared to their separate individual resonances. In this case, the coil that is tuned to the higher broadcast frequency tends to shift to higher frequencies and the coil that is tuned to the lower sensing frequency tends to shift to lower frequencies.
An EAS and/or RFID system operating at 13.56 MHz operation is of particular interest, considering the high field levels allowed by the FCC and other international regulatory agencies at this frequency. However, the bandwidth of the 13.56 MHz carrier signal allowed by the regulations is typically very small (e.g., about 14 kHz). For practical purposes, this dictates that the tag must be tuned to the essentially single-valued carrier frequency.
Generally speaking, the cost to manufacture two RLC circuits using pre-existing EAS and/or RFID tag technology is twice that of a single circuit. Accordingly, it is desirable to obtain a design and method of manufacturing a two (or more) coil circuit that costs essentially the same as that of a single coil circuit.
In commercial applications, an EAS tag/device typically deactivates its RF functionality when the article is legally purchased. This is generally referred to as a “write-once” single bit memory. If more bits of write-once memory were available in the EAS tag, store owners could determine, for example, whether an article had been purloined at a receiving dock (e.g., by an employee) or from a store aisle (e.g., by a customer). Thus, there are applications and a need for an EAS and/or RF tag having more than one bit of write-once memory.
Also, it is generally very difficult and costly to hold tight tolerances on the capacitor in an RLC resonant circuit of an RF electronic surveillance tag during a low cost manufacturing process. Accordingly, a capability or means for tuning the circuit after assembly would be advantageous so that capacitor manufacturing tolerances could be loosened and the overall process cost reduced.
One challenge to allowing reduced tolerances in low cost EAS tag manufacturing lies in matching the resonant frequency of an RLC circuit to a fixed frequency of an RF reader system, such as may be installed at the exit of a store or library, for example. As described above, this fixed frequency is generally very tightly controlled by the FCC (e.g., 13.56+/−0.01 MHz). Optimum tuning of an RLC circuit of the EAS tag would then require that the LC product be controlled to within 1.4%. It would also be desirable to have a design that allows the LC product tolerance to be substantially expanded to accommodate more cost-effective manufacturing processes.